Subtransient reactance is the relative measurement (%) of the internal impedance of a generator. For most common generators, its value ranges between about 15 and 20%. Customers often require generators with high subtransient reactance to limit fault currents during sudden short circuits.
The internal product specification for a new generator design usually specifies a minimum subtransient reactance (generally 14%) to ensure normal operation. The generator designer can usually design a generator to satisfy the product specification but sometimes at the cost of a larger generator.
Occasionally, a customer requests a subtransient reactance higher than what the generator was designed for. In such cases, the generator could be redesigned to meet the customer""s requirements, but the cost of doing so would be unacceptably high. There is a need, therefore, for a way to increase generator subtransient reactance without having to redesign the entire generator. It is also a requirement, however, that any such modifications not increase the losses, or reduce the generator efficiency. Generator armature temperatures should remain the same or be reduced, and the mechanical strength of the stator should be increased to minimize the vibration or winding motion.
Some previous attempts at increasing generator subtransient reactance include either depressing the coil sides in the slot, or increasing the number of turns to result in more flux linkages and, therefore, higher reactance. Both these approaches required redesign of the armature winding. The modified or redesigned winding may not have as much copper as possible, and so may have higher losses on temperatures.
In an embodiment of the invention, the armature wedges on units requiring higher subtransient reactance are replaced with molded wedges that have magnetic material embedded therewithin.
Subtransient reactance consists of six components: (1) armature slot leakage reactance; (2) zig-zag leakage reactance; (3) armature leakage reactance due to phase belts; (4) armature end winding leakage reactance; (5) armature leakage reactance due to peripheral leakage flux; and (6) equivalent subtransient rotor reactance per phase of stator direct axis. Among these, the armature slot leakage reactance accounts for about 40% of subtransient reactance. Therefore, an effective way for increasing subtransient reactance would be to increase the armature slot leakage reactance.
Conventional armature coil slot wedges serve to contain the armature bars in the slots. A wedge slide with a varying thickness is inserted between the armature wedge and the top filler to tightly hold all armature coil components inside the slot. By redesigning the armature wedge and/or the slide as a so-called xe2x80x9cmagnetic flux bridgexe2x80x9d, the armature slot leakage flux can be significantly increased and, in turn, the armature slot leakage reactance can be increased.
Thus, the invention provides a way to increase the leakage flux in the region between the top of the upper armature and the top of the slot without increasing the slot depth.
Accordingly, the invention is embodied in a magnetic assembly for being received in an armature slot for retaining armature coil components therewithin, the assembly having a longitudinal dimension generally parallel to an axis of the armature slot and a thickness dimension in a direction generally perpendicular to the longitudinal dimension and aligned in a depth direction the armature slot, the magnetic assembly comprising: a magnetic armature wedge structure including a molded body of a resin material having a magnetic material embedded therewithin, the magnetic material being embedded in the molded body so as to be disposed along substantially an entire length thereof. In the presently preferred embodiment, the magnetic assembly, further comprises a magnetic wedge slide adjacent the magnetic armature wedge structure, between the magnetic armature wedge structure and the armature coil components, the magnetic wedge slide being formed from resin having ferromagnetic particles distributed therethrough.